Re: APC vs. AFC: confused

Going by a very old data *** (~1998) and assuming you have SM48 which is listed as 0.4-0.47um for cutoff, then the following specs are listed:

center wavelength 0.48um
core 4um
cladding 125um
atten 50 dB/km
beat length 3mm
extinction ratio -30 db/100m


According to the data *** I have this was a special order fiber. There is also a high power version of at the same wavelength but all of the specs are identical. If the fiber is newer these specs may not apply.

P. Danek

JoS wrote:

Thank you for your reply.
Another thing I did not define well in this discussion was that we observe this pattern even when we used 514nm which is well above the cutoff of 470nm.
ad 1)As a source we use an Argon multi wavelength Laser. The beam is modified by an AOTF which selects the wanted lambda and turns the polarization by 90°. We had an old setup where the laser the AOTF and the coupler were in line. Now we changed to a setup where we employ two mirrors for the adjustment of the beam. So our source now is this laser then two mirrors then the AOTF and finally the coupler. So the beam should be polarized parallel to the breadboard and parallel to the slow axes. We changed the setup because the old one was not stable. And now with the new one we do not get much light through the fiber.
ad 2)Besides the lenses in the coupler (focal length: 3.5mm, achromat lens) there are no lenses in the optical path prior to the coupler.
ad 3) I have not been able to find out the numerical aperture and the attenuation coefficient of our fiber. It is a polarization maintaining fiber (PANDA). The length is 3m.
With the old setup we go much higher output powers after the fiber.

regards
reinhard

David M wrote:

Lets cover this again

You are operating a single mode fibre below its cut off wavelength.
This means that the fibre is no longer considered single mode, the
fibre is going to be cabable of supporting at leasty two modes. How
much power is in each mode will depend on many factors, and small
perturbations will cause cause coupling between the modes. To
complicate matters the same perturbations can cause coupling between
the the two polarisation states as well.

That pretty much takes care of the strange pattern on your screen and
the fact that it changes when the fibre is moved. There is no great
surprise there.

Your coupling efficieny on the other hand is execptionally poor. To
get an idea of how this can be improved it is useful to know
1. what is the source you are using and what are its output beam
properties.
2. what lens arrangement are you currently using..
3. what is the numerical aperture of the fibre. ( in fact what is the
fibre, how long is it and what is the attentuation coefficient)

I would expect that the coupling efficeincy can be improved by a
coupleof orders of magnitude, however this may be academic, if you are
looking for a stable output beam, given your current operating
wavelength.


cheers

David

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